Spec: Add a new virtual-customizable home attribute (#30917)

* Spec: Add a new virtual-customizable home attribute

* java: Use the new builtin home attribute

* python: Use the new builtin home attribute

var/spack/repos/builtin.mock/packages/attributes-foo-app/package.py

@@ -0,0 +1,12 @@
+# Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack.package import *
+
+
+class AttributesFooApp(BundlePackage):
+    version('1.0')
+    depends_on('bar')
+    depends_on('baz')

var/spack/repos/builtin.mock/packages/attributes-foo/package.py

@@ -0,0 +1,69 @@
+# Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack.package import *
+
+
+class AttributesFoo(BundlePackage):
+    phases = ['install']
+    version('1.0')
+
+    provides('bar')
+    provides('baz')
+
+    def install(self, spec, prefix):
+        if 'platform=windows' in spec:
+            lib_suffix = '.lib'
+        elif 'platform=darwin' in spec:
+            lib_suffix = '.dylib'
+        else:
+            lib_suffix = '.so'
+
+        mkdirp(prefix.include)
+        touch(prefix.include.join('foo.h'))
+        mkdirp(prefix.include.bar)
+        touch(prefix.include.bar.join('bar.h'))
+        mkdirp(prefix.lib64)
+        touch(prefix.lib64.join('libFoo' + lib_suffix))
+        touch(prefix.lib64.join('libFooBar' + lib_suffix))
+        mkdirp(prefix.baz.include.baz)
+        touch(prefix.baz.include.baz.join('baz.h'))
+        mkdirp(prefix.baz.lib)
+        touch(prefix.baz.lib.join('libFooBaz' + lib_suffix))
+
+    # Headers provided by Foo
+    @property
+    def headers(self):
+        return find_headers('foo', root=self.home.include, recursive=False)
+
+    # Libraries provided by Foo
+    @property
+    def libs(self):
+        return find_libraries('libFoo', root=self.home, recursive=True)
+
+    # Header provided by the bar virutal package
+    @property
+    def bar_headers(self):
+        return find_headers('bar/bar', root=self.home.include, recursive=False)
+
+    # Libary provided by the bar virtual package
+    @property
+    def bar_libs(self):
+        return find_libraries('libFooBar', root=self.home, recursive=True)
+
+    # The baz virtual package home
+    @property
+    def baz_home(self):
+        return self.home.baz
+
+    # Header provided by the baz virtual package
+    @property
+    def baz_headers(self):
+        return find_headers('baz/baz', root=self.baz_home.include, recursive=False)
+
+    # Library provided by the baz virtual package
+    @property
+    def baz_libs(self):
+        return find_libraries('libFooBaz', root=self.baz_home, recursive=True)